1. Field of the Invention
The present invention relates to an ultra-miniature, tag-use antenna used for a large-scale integration (LSI) chip and a tag using it for use in a radio frequency identification (RFID) system which is capable of carrying out a communication between a reader/writer and a tag by using a radio high frequency signal.
2. Description of the Related Art
An RFID system is one for transmitting a signal of approximately one watt from a reader/writer (simply “RW” hereinafter), receiving the signal on a tag side and retransmitting a response signal back to the RW side, by using an ultra high frequency (UHF) between 860 MHz and 960 MHz, thereby enabling the RW to read information stored in the tag.
The tag is constituted by a tag-use metallic antenna formed on a flat surface such as a sheet or a film, et cetera, of approximately 0.1 mm thick, and an LSI chip connected to a feed point of the tag-use antenna. The LSI chip is usually smaller than a sesame seed, actually approximately 0.2 mm thick and its area size is approximately 1 mm square.
A communication distance between the RW and tag is about 3 to 5 m, depending on a gain of a tag-use antenna, an operation voltage of a LSI chip, and an environmental condition of the surrounding, et cetera.
FIGS. 1A, 1B and 1C are diagrams respectively describing tag-use antennas used for a conventional RFID system. FIG. 1A shows a tag-use antenna comprising dipole parts 2 extending horizontally on both sides of a power feed part 1; FIG. 1B shows one having folded dipole parts 3 having both ends of FIG. 1A turned back; and FIG. 1C shows one having an inductance part 4 connected in parallel with the dipole parts 2 to the feed part 1 shown in FIG. 1B.
FIG. 2 is a diagram showing an equivalent circuit of the tag-use antenna and LSI chip used for an RFID system, with the left side showing an equivalent circuit 5 of the tag-use antenna and the right side showing an equivalent circuit 6 of the LSI chip.
FIG. 3 is a diagram exemplifying an analysis, by an admittance chart, of a tag using a conventional tag-use antenna. An admittance chart is indicated by zero (“0”) ohm on the left side of a pure resistance line, which divides the circle of the chart into the top and bottom half, and infinite (“∞”) ohms on the right side thereof.
As shown in FIG. 2, a tag-use antenna can be equivalently expressed by a parallel connection of an emission resistance Ra and of an inductance La, while an LSI chip can be equivalently expressed by a parallel connection of a resistor Rc and of a capacitance Cc.
Then, the parallel connection of the tag-use antenna and LSI chip makes the inductance La and capacitance Cc resonate, and they match at a desired resonance frequency f0 as is apparent from an expression “f0=1/(2π√(LC) )”, resulting in a reception power at the tag-use antenna being adequately supplied to the LSI chip side.
That is, letting an emission resistance Ra of the tag-use antenna be 400 ohms for example, a resistance Rc of the LSI chip be 500 ohms, for example, a configuration be so as to cancel out resistance of both, and assuming L=La=20 nano Henry (abbreviated as “nH” hereinafter) and C=Cc=1.4 pF in the above noted expression of the resonance frequency, then a desired resonance frequency of f0=953 MHz required for an RFID system is obtained.
For a basic antenna used for a tag-use antenna, first conceivable is a dipole antenna of a whole length of about 145 mm which is constituted by dipole parts 2 extending horizontally in both direction of the feed part 1 shown by FIG. 1A.
In this configuration, the feed part 1 connected to the dipole parts 2 extracts a power from a signal received at the dipole parts 2 and feeds the power to the LSI chip equipped on the feed part 1 and also transfers the signal per se to the LSI chip. The configuration of the dipole antenna actually measures an emission resistance Ra=72 ohms.
Incidentally, impedance of an LSI chip of the above noted resistance Rc=500 ohms and capacitance Cc=1.4 pF is indicated at a position diagonally on the right below in the direction of about “−40 degrees” of an ωC zone in the admittance chart (FIG. 3 shows the position simply by a circular plot pointed as “chip”).
In this case, an optimum position, in the admittance chart, of the dipole antenna resonating with the above described LSI chip is a position of the LSI chip symmetrically reversed relative to the pure resistance line of the admittance chart, and FIG. 3 shows the position diagonally on the right above in the direction of about “+40 degrees” of an ωL zone.
This position is one for an impedance with an emission resistance Ra=500 ohms and an inductance La=20 nH (FIG. 3 shows the position by a circular plot indicating “the most optimum position”).
As such, an emission resistance Ra required for an RFID tag-use antenna corresponding to an LSI chip of resistance Rc=500 ohms and capacitance Cc=1.4 pF is very high, i.e., about 500 ohms, and therefore the emission resistance Ra=72 ohms of the dipole antenna shown by FIG. 1A is far too small.
It is accordingly necessary to increase an emission resistance Ra up to about 500 ohms by devising a configuration of the dipole antenna. Then devised is a folded dipole antenna having a folded dipole part 3 of a whole length of 145 mm folding back from the both ends of FIG. 1A, as shown by FIG. 1B.
This configuration makes it possible to increase an emission resistance Ra. This configuration is known to allow setting of emission resistance in the range of about 300 to 1500 ohms, depending on a wire width of the folded part.
FIG. 3 shows an impedance position of a folded dipole with an emission resistance Ra being 400 ohms indicated by a triangle on the pure resistance line.
Here, with the emission resistance Ra being maintained at 400 ohms, a further connection of an inductance part 4 to the feed part 1 of FIG. 1B parallelly with the dipole part 2 as shown in FIG. 1C rotates the antenna characteristic counter-clockwise on the admittance chart.
This results in positioning, close to the most optimized position, the antenna characteristic of the folded dipole antenna, connected with the inductance L, having a resonance frequency of 953 MHz as shown by a triangle as the folded dipole connected with the inductance L (the “L-connected folded dipole antenna” hereinafter) in the ωL zone of FIG. 3.
The admittance chart shown by FIG. 3 exemplifies characteristic between 700 and 1200 MHz. In the range of the resonance frequency, it is apparent that the antenna characteristic locus 7 of the L-connected folded dipole antenna circles around the resonance most optimum value (i.e., the most optimized position of Ra=500 ohm and La=20 nH).
That is, it is apparent that the configuration of the L-connected folded dipole antenna shown by FIG. 1C widens a frequency band resonating with the LSI chip.
Incidentally, an RFID is used by being attached to various bodies as a tag. In the case of such a body being a styrofoam, the dielectric constant ∈r of the RFID is approximately 1.1 which is about the same as the value in the air (∈r=1).
That is, in the case of attaching a tag onto a styrofoam, it becomes about the same as floating the tag in the air.
In the case of a body attached with an RFID being a plastic for example, an effective dielectric constant around the antenna becomes large if the thickness of the plastic is 2 mm, since the dielectric constant ∈r of a plastic material is about ∈r=3.
Meanwhile, a behavior of the RW communicating with an RFID at the operating frequency 953 MHz is empirically known to be approximately the same as the characteristic at 953 MHz in the air displaced by 100 MHz.
As such, a practicality is hampered if a communication distance of the antenna fluctuates when being attached onto various kinds of bodies, that is, when the operating frequency is displaced, and therefore desired is an antenna whose communication distance does not change even if it is attached to various kinds of bodies.
Therefore, a good antenna for an RFID is one capable of having a wide frequency band characteristic, that is, having a wide frequency characteristic.
The L-connected folded dipole antenna, shown by FIG. 1C, comprising the antenna characteristic as shown in FIG. 3 has an adequately wide band, e.g., the bandwidth of the one rotation part 7a according to the antenna characteristic locus 7 of FIG. 3 is approximately 200 MHz, can be characterized as a good antenna whose communication distance being hard to fluctuate due to a material to be attached to (i.e., uninfluenced by a material to be attached to).
However, there is a strong demand by users for miniaturizing the RFID. An antenna with a size of 145 mm horizontal by 15 mm vertical is too large for a tag use. It may be just possible to use it for managing a book for example, while there is no degree of freedom by a limitation of its usage in terms of other practicalities, thus requiring further miniaturization.
Incidentally, if one tries to confine the entire size of an antenna to 80 by 20 mm for example, he must bend the antenna line in a serpentine fashion (or “meandering”) in order to house the elongated line length into a small area size.
It is, however, known that a miniaturization of an antenna widens a frequency interval (e.g., the one rotation bandwidth becomes approximately a mere 20 MHz) of the characteristic part (i.e., the characteristic locus 7a) that rotates one revolution as shown in FIG. 3.
That is, the miniaturization of an antenna narrows a frequency band. In other words, an RFID comprising such miniaturized antenna changes communication distances drastically depending on the material to be attached to. This is faced with practical problems.